It is known that in order to penetrate the mammalian egg zona pellucida, sperm must first undergo a fusion and vesiculation of part of the sperm head plasma membrane and the sperm outer acrosomal membrane. This acrosome reaction normally occurs as part of the capacitation of sperm afterseveral hours in the female tract. However, it can also occur under certain in vitro conditions. A hamster sperm in vitro acrosome reaction inducing system is being used as a model for the in vivo event. Studies are being made of molecules in bovine follicular fluid which can cause the acrosome reaction in hamster sperm. Other studies include an investigation of the effect of certain membrane agents upon the acrosome reaction. It is hoped that this work will increase our understanding of the biochemical basis of the acrosome reaction.